Concrete Experimentation

Concrete Doors

Wood and metal (steel, aluminum) are the most common materials used in the construction of door and window frames and for the doors themselves. These materials are subject to decay or rust and require extensive maintenance, especially in coastal areas where wind-driven rain, high humidity, salt spray and strong sun subject buildings to extremely adverse conditions. In addition, quality wood is expensive, especially in Greece where it is in short supply, and its extensive use raises questions of impact on the environment.

Investigations of alternative materials in the production of doors, door frames and window frames have not heretofore included concrete, primarily because of its weight. Recent developments in concrete technology, however, suggest its potential. New cements are capable of giving concrete much greater strength. Lightweight aggregates and air-entraining agents reduce the weight of the material. Cement additives reduce moisture permeability. These qualities have been explored extensively within the construction industry. Reinforcement technology is also advancing. Engineers in the field of textiles are developing glass and carbon fiber materials with strengths comparable to the mild steel commonly used in reinforced concrete. The use of textile reinforcement could eliminate problems caused by steel corrosion.

Christos embarked on research, while serving as department head of architecture at North Carolina State University, into concrete doors, He produced four prototypes and a monograph, Concrete Doors: An Investigation into Alternative Materials, which detailed the design process and methodology. And it was a starting point for continuing the investigations on Kythnos, where we embarked on the task of replacing all wood doors and frames with ones cast in concrete.

In the initial investigations of the potential for using concrete for doors and door frames, manageable weight was a prime consideration. Aesthetic appeal, added security, and water and heat resistance were also objectives. The initial experiments conducted at NCSU were extremely promising. The four concrete doors that were cast and mounted on vertical pivots in a public space generated considerable interest among School of Design faculty and students, the professional community, and representatives from the cement/concrete industry.

The results encouraged us to proceed with our plans to replace all wood doors, door frames, window frames and shutters on Kythnos with ones cast in concrete. However, since our time on the island was limited to brief summer visits, the pace was slow. The first concrete door frame was installed in 1997. Its accompanying door followed shortly thereafter. The rate of replacement soared after our retirement and relocation to Greece in 2001 and by the end of 2007, all wood frames and doors had been eliminated and cast concrete ones installed. There are 12 doors and 20 shutters, plus a dozen or so access panels and trapdoors.

The first concrete doors replaced small, bulkhead doors (about 1.70 x 0.70 meters). Using regular-weight aggregates, they weighed 100 kg. and did not seem to present any problems in hanging or swinging. Thus made brave, we abandoned the thought of lighter aggregates (and possible loss of strength), and we now have produced doors weighing up to 150 kg., whose only problem is the difficulty in handling them between mold and installation.

Process

Design and Construction of Molds:

Each door frame requires three to four pieces: head, left and right jamb and threshold for some doors; window frames are four pieces each, allowing for open mortise and tenon joints at the corners, as in wood technology.

Christos designed and constructed molds (sometimes referred to here as forms) in his woodworking shop in the U.S. and later in Greece, using a radial arm saw and a variety of hand-held tools. Molds for the first door frames were constructed while we still lived in the U.S. They were designed to be transported in duffel bags — the jambs, for instance, were made in two pieces. Duffel bags filled with odd pieces of plywood gave fits to suspicious customs officials who couldn't figure out what it was we were really smuggling

Each of the four molds for the window and door frame pieces, and each door and shutter form is designed as a tray, requiring five pieces of plywood: one for the bottom and four for the sides. Typically, the two long sides of the mold are hinged to the bottom for speed of assembly/disassembly, while the two short pieces are joined to the long ones and to the bottom using bolts and barrel nuts (the kind used in Ready-To-Assemble furniture) to allow for strong joints and repeated use of the mold.

Preparation of Molds for Casting:

After the molds have been assembled, with screw anchors and other relevant parts in place, they receive a coat of oil that acts as separator. The ends of the anchors, which are typically open, are plugged with plasticine to prevent cement slurry from filling them up.

The first doors were reinforced with polypropylane strands and/or short pieces of cold drawn carbon steel wire mixed in the concrete. In later versions, however, we began using a galvanized wire mesh in conjunction with the polypropylane strands, as our fears of corrosion were alleviated by the use of a latex cement additive, which decreases water penetration, and by primers and paints that appear to seal surfaces quite well. Time will tell.

Concrete Batch:

Typically, several molds are filled in a casting operation. The volume of required concrete is calculated and materials are measured out in advance, to minimize time between batches. Our concrete mixer is relatively small, capable of mixing batches of about 50 liters, sufficient for most doors.

The ratio of materials is as follows:

• Aggregate (quarry sand) to cement = 3:1

• Liquid to dry mix (sand plus cement) = 1:4 with variations to allow for wet sand

• The liquid comprises a mixture of water and latex cement additive in a ratio of 3:1

• Polypropylane strands, about a fistful, are added to each batch

With our concrete mixer, the mixing process that seems to work best is to place the liquid and the polypropylane strands in the mixer first along with a shovelful of sand and then, with the machine running, add the cement gradually so as to produce a slurry. Finally, add sand, also gradually, and run the batch for about five minutes to assure uniformity.

The installed frames and doors or shutters are coated with a sealant/primer and painted with two coats of a latex paint, specially formulated for use on concrete.The installed frames and doors or shutters are coated with a sealant/primer and painted with two coats of a latex paint, specially formulated for use on concrete.

Carrying the idea a step further, Christos decided to design and produce a special insulated for our bedroom, described here.